Process for preparing alumina wool



Sept. 12, 1967 R. H. KELSEY 3,341,285

PROCESS FDR PREPARING ALUMINA WOOL Filed July 16, 1964 L IS INVENTOR Robert H. Kelsey BY Z a /m4 ATTORNEY United States Patent 3,341,285 PROCESS FOR PREPARING ALUMINA WOOL Robert H. Kelsey, West Acton, Mass., assignor to Horizons Incorporated, a corporation of New Jersey Filed July 16, 1964, Ser. No. 383,097 1 Claim. (Cl. 23-142) ABSTRACT OF THE DISCLOSURE This invention describes a method for growing alumina wool from the vapor evolved from a melt of aluminum by controlling the flow of dried hydrogen passing over the melt,

This invention relates to the preparation of alpha alumina in the form of extremely fine wool-like fibers possessed of appreciable strength at high temperatures. More particularly, it relates to an improvement over the processes describedin United States Patents 3,023,115 and 3,077,380 issued on Feb. 27, 1962, and Feb. 12, 1963, respectively, to Eugene Wainer and Andrew L. Cunningham.

Either of two alumina products is obtained, as described in the above noted US. patents when relatively pure hydrogen gas containing minute and critical amounts of water vapor is passed over the surface of molten aluminum or a molten aluminum alloy in an apparatus in which the vaporous products evolved from the melt (under the conditions prevailing in the apparatus) are permitted to contact a surface formed of at least one oxide having a heat of formation less than the heat of formation of A1 0 at the prevailing temperature. The two products are identified as wool and as Whisker and typical dimensions of the two types are approximately as follows:

Diameter (microns) Length (inches) Wool Whisker 0 At least 80 As described in the above noted patents, both wool and whisker types of fiber growth have been observed. The first fibers deposited by condensation of the aluminum oxide smoke under the preferred conditions of operation described above, appear round in cross section, and have diameters ranging from less than one micron to about six microns. The appearance of the fine diameter product first formed is very similar to that of a carded wool or short fiber cotton. These fibers tend to be short lengths although occasionally a long fiber, several inches in length occurs.

The other type of fiber observed is a much larger brittle form, seen usually as needles growing out from the melt and the melt container. These relatively coarser, more brittle fibers appear to be formed from the finer, woollike fibers when those fibers are subjected to temperatures in excess of some limiting value, particularly for extended intervals of time, or where the desired refractory source of oxygen is not properly available.

It has now been found that even when the prevailing reaction conditions indicated to be essential in US. 3,077,380 are maintained, the desired wool-like product is not always obtained, and that often a run in which aluminum wool is the first product collected, gradually produces increasing amounts of the less desirable coarse, brittle whisker products.

A principal object of this invention is to provide a process in which the entire product is the wool-like form of alumina and which may reproducibly produce the'same product with virtually no contamination by the coarse productin run after run.

This result is obtained by maintaining the reaction conditions as specified in the above noted US. patents except that the rate of flow of hydrogen over the surface of molten aluminum is controlled With critical limits, defined below.

The apparatus is depicted schematically in the drawings in which- FIGURE 1 is a view, partly in section, of an apparatus similar to that shown in Figure 1 of United States Patent 3,023,115; and

FIGURE 2 is a section thereof taken on plane 2-2 of FIGURE 1.

As shown the apparatus consists of a refractory tube 10 two inches in diameter, between 36 and 38 inches long with a W inch wall thickness The middle portion of this tube is enclosed in an electric furnace 12 capable of heating and maintaining the tube and its contents at temperatures up to about 1500 C. A siliceous boat 14 positioned in tube 10 contains charge 16 consisting of a mixture of Al powder and finely divided titanium metal. An inlet at one end and an outlet at the other are provided for the flow of hydrogen through the tube. A second boat 20 is adapted to be supported above boat 14 and serves as a preferred condensing surface for the deposition of alumina wool thereon.

Operating with an apparatus such as that described above and in US. Patent 3,077,380 and U8. Patent 3,023,115, i.e. utilizing a tube approximately 36 inches long and 2 inches in diameter, with a wall thickness of inch and one or more fire brick (Al O .SiO boats 9 inches long, 1% inch outside diameter and /5 inch Wall thickness, containing a charge of powdered aluminum such that when melted, the top surface of the melt contains about 5 square inches of area, it has been found that the rate at which the moisture-containing hydrogen flows through the tube is critical and that wool growth is localized and enhanced by employing a second boat as a cover.

At hydrogen flow rates above two liters per minute, little fiber is grown, possibly because the gases from which it derives the oxygen are swept away too rapidly to permit an adequate concentration to be developed. At hydrogen flow rates below the one liter per minute exemplified in US. Patent 3,023,115, the suboxide concentration is increased, and a mixture of wool-like fiber and coarse fiber is produced. This product is obtained down to flow rates of about 100 mL/minnte.

The flow rates indicated above refer to volumes of hydrogen computed at standard conditions of pressure (760 mm. Hg) and temperature (0 C.) although the actual pressure and temperature of the hydrogen are generally other than standard. A flow rate of 60 mL/min. means an amount of hydrogen equal to 60 ml. of hydrogen measured at standard temperature and pressure is exposed to the 5 square inches of melt surface area every minute.

In the present invention the production of only the wool-like product is insured by utilizing hydrogen flow rates between about 60 mL/minute and ml./minute in the apparatus specified. Usually the hydrogen enters the ceramic tube at room temperature and at a pressure of about 3 psi. This appears to provide enough flow to hold the concentrations at a sufiiciently low level to permit an orderly growth of wool fiber, without risk of exhausting the supply of water vapor or of hydrogen, and at the same time avoids the formation of the brittle coarse whiskers which occurs at higher flow rates.

The following example will serve to further illustrate the invention:

Ten grams of a mxiture of powdered aluminum (120 mesh) and powdered titanium are placed in a siliceous boat of the size described above and inserted in a mullite furnace tube. The tube is closed, and a flow of nitrogen at a rate of one to two milliliters per second is begun. The furnace temperature is raised to about 1800 F. (1000 C.), at which time the nitrogen flow is stopped and hydimensions as heretofore specified.

Having now described this invention in accordance with the patent statutes it is not intended that it be limited except as required by the appended claim.

I claim:

In the process of forming fibrous anhydrous, alpha alumina which consists in heating a metal consisting essentially of aluminum to melt said metal; contacting the melt with gas consisting essentially of hydrogen gas dried to a dewpoint between minus C. and minus 90 C., while said melt is maintained at a temperature of at least 1370 C. to below about 1S10 C.; providing a condensing surface in physical contact with the vapor phase above the melt, said surface containing at least one refractory oxide having a heat of formation smaller than the heat of formation of aluminum oxide, at the prevailing temperature; and forming said fibrous alumina on said condensing surface; the improvement which comprises: maintaining the rate of flow of said gas between about nil/min. and ml./min. for a melt surface area of about 5 square inches thereby producing alpha alumina substantially entirely in the form of wool-like fibers.

References Cited UNITED STATES PATENTS 3,011,870 12/1961 Webb et a1. 23-142 3,023,115 2/1962 Wainer et al 106-65 3,077,380 2/19'63 Wainer et a1. 23142 3,147,085 9/1964 Gatti 23-142 X FOREIGN PATENTS 608,032 11/1960 Canada.

OSCAR R. VERTIZ, Primary Examiner.

H. T. CARTER, Assistant Examiner. 

